Page:Weather Facts and Predictions.djvu/20

 radiation both solar and terrestrial, causes the heat of the sun to fall upon tourists with scorching effect.

When the air is what we call sultry, it is saturated with moisture, and evaporation from our bodies proceeds sluggishly. Hence the well-known oppressive sensation often felt in the summer months, especially in July. Under those circumstances there will be very little, if any, difference between the readings of the wet and dry bulb thermometers.

At any time when the air has been for a while much heated above the usual temperature of the season, a sudden squall, with or without rain, will often come on without much warning.

A period of excessive cold is often followed by destructive gales of wind.

In the northern hemisphere the thermometer rises with E., S.E., and S. winds. With a S.W. wind it ceases to rise, and begins to fall: it falls with W., N.W., and N. winds; and with a N.E. wind it ceases to fall and begins to rise.

The thermometer (shaded from the sun and freely exposed to the air) when much higher between 8 and 9 a.m. than the average, indicates southerly or westerly wind (equatorial); but when considerably lower, northerly (Polar) currents.

The average temperatures at Greenwich in the shade and exposed in air are nearly the mean temperatures of each 24 hours, taking the year through, around London. Making an allowance for the difference between the mean annual temperature of Greenwich and of any